This is an improvement over the attachment assembly for compressor blades that is described and claimed in U.S. Pat. No. 5,022,822 granted to S. M. Sincere on Jun. 11, 1991 entitled "Compressor Blade Attachment Assembly" and commonly assigned to United Technologies Corporation, the assignee of this patent application. In this patent the objective is to include as many load transfer lugs as possible with the goal of reducing the dead rim material.
As one skilled in this art will appreciate the size of the disk supporting the blades of the rotor, be it the compressor rotor or the turbine rotor, is essentially predicated by centrifugal effects on any mass that is not continuous about the circumference of the disk. The mass which is not continuous, as one skilled in this art knows, is considered to be dead load, and obviously, dead load is undesirable from structural and rotor performance standpoints. The U.S. Pat. No. 5,022,822, supra, teaches that unless the blade and disk are integral, the larger the number of grooves in the disk that can accommodate the tongues of the blades within the confines of the width of the blade, the lower the dead rim area and hence the lower the dead load, resulting in a reduction in dead rim material with a consequential weight savings of the rotor. Obviously, if one were to adhere to these teachings, one would include as many grooves in the disk as possible and a corresponding number of tongues in the blade to maximize on the weight savings characteristics of this concept. To carry this one step further, it is always important to reduce engine weight to enhance engine performance.
The fallacy with these teachings is that the multi-lug (another term for the expression "tongue") theory focuses only on minimizing the disk dead rim material and not on minimizing the system's dead load. Accordingly, in order to achieve an optimum attachment design, consideration not only must be given to the dead rim area of the disk, but also to the dead load of the blade, the attachment and the platform, namely, the system's dead load. As will become evident from the description that follows, this invention not only accounts for the dead rim area of the disk but it, also, considers the dead load associated with the attachment and platform, which heretofore has never been considered. While the absents of such considerations may be acceptable for a solid blade, such as that utilized in the fan and/or compressor of a gas turbine engine, it is not appropriate for air cooled turbine blades, especially for low pressure turbine blades with shallow broach angles.
It still holds true that by increasing the number of load attachment locations (i.e. blade attachments) on a turbine engine rotor will result in a more evenly distributed loading pattern on that rotor. Also, for a given amount of rim pull associated with a rotor, the greater the number of attachments, the smaller the resulting dead rim weight, and consequently a lighter rotor package. While the concept of placing multiple attachments on to a single blade is a basic premise behind the multi-lug attachment, I optimize this concept by considering the overall system dead load and not restricting the design to incorporating the maximum number of attachments that could be packaged in the disk rim area. According to my invention the airfoil to blade attachment is an essential ingredient in the design of the blade attachment. To this end, I have found that the ideal attachment for air cooled turbine blades and especially for low pressure turbine blades with shallow broach angles, is to provide a two-lug attachment with the attachment configuration being judiciously selected.
The two-lug attachment of my invention provides a direct load path and hence, achieves optimum load transfer and minimum dead load. In a certain application it was found that while the dead rim area of the disk was actually slightly increased in comparison to a three lug attachment design, the total dead area of the overall system, i.e. including the attachment and platform, was reduced. By achieving a direct transfer of load from each airfoil wall, namely the pressure side and the suction side, to the corresponding attachment lug, the distribution of load is improved, gaining the benefits of a multi-lug attachment while enhancing the load distribution on the rotor. This obviously results in a lighter blade attachment and a lighter disk in comparison with conventional firtree and dove tail attachments and thusly, decreasing the overall weight of the engine.
As mentioned above, in accordance with this invention, the two-lug concept allows a direct transfer of load from each airfoil wall down into the corresponding attachment lug. Each lug transitions over the neck height (i.e. the distance from the platform to adjacent the bottom edge of lug of the blade) of the attachment from a curved airfoil contour down to a straight rail at the disk rim. In essence, each leg of the attachment is a continuation of either the pressure side or suction side wall of the airfoil. Each lug transitions from a curved contour down to a straight contour and terminates at the disk rim as two parallel dovetail attachments. It also tapers in thickness from 0.120 inches to 0.160. This taper allows for optimization in two parameters, namely, 1) the reduction of weight of the blade attachment, and 2) the blade attachment tensile stress.
In comparisons made by well known structural analyses technique, the load distribution along the lug on the two-lug design has proven to be better than the load distribution associated with the heretofore well known firtree attachment design as well as that of a three-lug attachment.
It has been found that the weight savings can be attributed to both the blade attachment itself and the disk. The weight savings attributed to the blade attachment comes from the reduced mass associated with the attachment area. The weight savings associated with the disk comes from the fact that the entire dead load (disk rim and attachment) is significantly reduced.
Appreciation of this invention can be realized when considering that the amount of weight savings as determined by the structural analysis, of both the blades and disk of a rotor comprising 52 blades designed for a high speed, high performance military aircraft. A total of eight (8) pounds was saved in the blades alone in comparison with conventional firtree design. The disk over the conventional firtree design realized a reduction in seventeen (17) pounds for a combined weight savings of twenty-five (25) pounds for the rotor system.